The Remarkable Work of

Vaccination and Your

Horse’s Immune System

Your horse’s immune system does extraordinary

and complex work, but even this intricate defense

mechanism can’t go it alone. Vaccination has proven

to be an extremely effective and safe way to provide

a targeted boost to the immune system to help safeguard

horses against deadly infectious diseases.

Unfortunately, there is a lot of misinformation and

myths circulating about vaccination. Belief that vaccination

is unnecessary if the disease is rarely seen in a

particular region, or farm; natural immunity is better;

or concern that vaccines can overload the immune system,

are just a few. These untruths are often perpetuated

by the Internet.

A better understanding of the equine immune system,

along with how vaccines enhance immune responses

can be helpful in dispelling these myths and

B y E a r l G a u g h a n , D V M , D A C V S

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Copyright © 2016 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.Used with permission.

improving the overall health and welfare of our equine

companions. The relationship you have with your veterinarian,

who is your partner and your horse’s best line

of defense against infectious disease, is a vital link to

making the best health care decisions for your horse.

The basics of immunology

Immunity is the body’s ability to recognize and dispose

of foreign substances that can be harmful. There

are many ways to classify immunity, but the two major

components of the immune system are natural and acquired


1. Natural immunity (also known as native or innate

immunity): Fast-acting, non-specific immune response

that occurs at the site of infection and is

always present and readily activated. This is the

horse’s built in, first line of defense that is present

from birth. Non-specific chemicals and cells are

the major components of the natural immune response.

Examples of mechanisms responsible for

natural immunity include certain types of white

blood cells, proteins, antimicrobial compounds,

and even natural physical barriers, such as the

skin, mucous membranes and stomach acid. Animals

and people have some degree of natural immunity

to almost all infectious agents.

2. Acquired immunity (also known as adaptive immunity):

Antigen-specific, acquired immunity that is

induced and remembered after exposure to specific

antigens. It occurs after initial exposure to a specific

pathogen and requires several days to weeks to develop.

Acquired immunity consists of two parts:

a. Passive immunity, such as achieved through

the feeding of colostrum (first milk-containing

antibodies) to a newborn foal and/or

through giving plasma containing antibodies.

b. Active immunity, which occurs following exposure

to an infectious agent or to a vaccine.

There are two types of active immune response:

i. Humoral immunity (production of antibodies).

Examples include the antibody response

to tetanus, rabies and other vaccinations.

ii. Cell-mediated immunity (CMI). An example

of CMI would be a response that does not involve

antibodies. What occurs, instead, is the

release of cytokines (proteins that activate numerous

components of the immune system)

that help direct cell function in response to an

antigen or vaccination. This type of response

is generally thought to be more robust.

Both types of immune responses work in tandem to

protect the horse from invading pathogens. For example,

acquired immunity relies on the natural (or innate)

immune system to communicate information about

the type of pathogens potentially invading the body.

This, in turn, helps the united immune system mount

the appropriate and most profound response.

Vaccines and the immune system

Killed versus modified-live vaccines

Different types of vaccines stimulate the immune system

differently. “Killed” (inactivated) vaccine antigens

are processed and recognized by the immune system

as exogenous antigens – those having originated externally.

Killed vaccines primarily induce an antibody

response, but a less aggressive CMI response also occurs.

“Live” (modified-live) vaccine antigens have been

altered (attenuated) so they can no longer cause clinical

disease. Modified-live vaccines are recognized and processed

by the immune system as endogenous antigens –

having originated within – which mimic how a natural

viral or bacterial infection is processed. These antigens

elicit both antibody and strong CMI responses.

Route of administration can also have an effect on

the immune response to vaccination. For example, intranasal

modified-live vaccines have an enhanced ability

to stimulate the natural (innate) immune response in

conjunction with stimulation of an adaptive (acquired)

immune response.

A study conducted at Colorado State University

demonstrated how the modified-live intranasal influenza

vaccine (marketed as FLU AVERT® I.N.) stimulates

a non-antigen specific, innate immune response

at the site of respiratory infections – the upper respiratory

tract. This non-specific immune response may

help provide some degree of protection against not

only equine influenza virus (EIV), but other respiratory

pathogens as well.

Monovalent versus combination vaccines

Combination vaccines for horses have changed the way

veterinarians approach vaccination. Combination vaccines,

which include multiple disease antigens in one

injection, help eliminate the need for multiple injections.

Most cover the majority of core vaccine recommendations,

as well as important risk-based diseases, such as

equine influenza and herpesvirus. Monovalent (containing

one antigen) vaccines are still used frequently

and work well, but do not necessarily improve protection

over combination vaccine products.

We often get asked if each fraction (antigen) within

these combination vaccines is proven to be effective. In

other words, is each fraction (disease antigen) in a combination

vaccine proven to provide protection against

Copyright © 2016 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.Used with permission. | Issue 4/2016 2



that disease antigen in the combination vaccine? The

answer is yes! Effectiveness requirements for licensing

of multiple antigen vaccines (combination vaccines)

are the same as those for single antigen (monovalent)

vaccines. So we know that combination equine vaccine

products are effective in preventing and lowering the

risk of the diseases addressed by each vaccine.

However, depending upon the antigen, there can

be differences in testing (e.g., challenge studies, titer

responses, lab animal studies, etc.), which is why you

may hear that not all killed vaccines are the same.

For example, both the West Nile virus (WNV) fraction

and the equine influenza virus (EIV) fraction of

Merck Animal Health vaccines have been proven effective

following live challenge with the disease causing

agent. Following vaccination with Merck Animal

Health’s largest combination equine vaccine (Prestige®

V + WNV), a live intrathecal challenge study was used

to demonstrate the effectiveness of the WNV fraction,

and a live influenza challenge study was used for the

EIV fraction. These challenge studies help demonstrate

protection in the face of natural exposure (as a

horse would normally encounter disease). Not all vaccines

or antigens are licensed using challenge studies.

Some rely on serology or titers only (antibody).


The following are considered “core” vaccinations because they protect from

diseases that are endemic to a region, those with potential public health

significance, required by law, virulent/highly infectious, and/or those posing

a risk of severe disease or death.

• Eastern Equine Encephalomyelitis (EEE)/Eestern Equine

Encephalomyelitis (WEE)

• Rabies

• West Nile Virus (WNV)

• Tetanus

In addition to the core vaccines that every horse should receive, risk-based

vaccination protocols vary according to your horse’s specific needs, and

should be directed by a veterinarian. Considerations include the horse’s age,

environment, use, risk of exposure to infectious agents, and geographic

location the horse lives in or may travel to. Risk-based vaccines include:

• Equine Herpesvirus (EHV)

• Equine Influenza Virus (EIV)

• Strangles (Streptococcus equi)

• Potomac Horse Fever (PHF)

• Equine Viral Arteritis (EVA)

• Rotavirus

• Anthrax

• Botulism

The immune system cannot be overloaded

The remarkable work of the immune system is showcased

by its ability to respond to literally thousands of antigens

every day. Equine immunologist David Horohov,

PhD, chair of the Department of Veterinary Science at

the University of Kentucky College of Agriculture, Food

and Environment and director of the Gluck Equine Research

Center in Lexington, Ky., said it is unlikely that a

combination of antigens in a vaccine product would exhaust

or compromise the immune response.

“Every day, the horse, like all animals, encounters a

multitude of foreign antigens in its environment. The

body has evolved a number of protective mechanisms

to deal with this exposure. While vaccination does

provide a concentrated exposure to specific antigens,

and even if multiple vaccines are administered, one

would not expect any untoward effect on the immune

system because of these mechanisms.”

Adjuvants and preservatives are safe

An adjuvant is a substance included in many inactivated

and some modified-live vaccines to enhance the immune

response to specific antigens. Killed vaccines generally

require an adjuvant to enhance the vaccine antigen presentation

to the horse and elicit an efficacious immune

response. Many adjuvants have the ability to help elicit

both humoral (antibody) and cellular response. Most adjuvants

are derived from safe chemicals, microbial components

and/or mammalian proteins.

“The increased efficacy of most equine vaccines

over the past 20 years has been almost entirely due

to the development of more effective adjuvants,” Dr.

Horohov said. “Adjuvants are used to enhance the immunogenicity

of the vaccine by both prolonging the

exposure of the immune system to the antigen and by

providing necessary co-signals through the interaction

of the adjuvant with antigen presenting cells.”

Adjuvants are generally considered safe – nonallergenic,

non-toxic and non-antigenic. However,

adjuvants can create local inflammation in the tissue

of some horses. This most commonly results in injection-site

reactions (bumps, swelling), fever, local soreness,

lethargy and reduced appetite.

Vaccine manufacturers place a great deal of importance

on research and development efforts, and safety

testing to determine the appropriate balance of adjuvant

to antigen to elicit an optimal immune response

with the least potential reactivity.

Merck Animal Health vaccines contain the proprietary

Havlogen® adjuvant, which is recognized for low

reactivity and its delayed release properties. Merck

Animal Health vaccines are manufactured using an

advanced filtration process to ensure safety, purity, re-

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Copyright © 2016 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.Used with permission.

liability and predictability of a positive response.

Most vaccines also include some type of preservative

to increase shelf-life, for safe storage, and to prevent

bacterial and fungal contamination that could

compromise the vaccine. One such preservative is thimerosal,

a mercury compound that is used in the majority

of killed equine vaccines in the marketplace. It is

a well-established antiseptic and antifungal agent that

has been widely used in human and animal vaccines

for decades. Thimerosal is considered extremely safe,

with only minor, local reactions at the injection site occasionally

appearing. Systemic and long-term negative

results have not been associated with thimerosal use.

Speculation that thimerosal may be associated with

autism and other health related problems in humans,

primarily children, has not been scientifically founded.

“The concentration of these preservative materials

is quite small,” added Dr. Horohov. “Once injected

into the horse, their effect is diluted nearly to the point

of extinction. While a great deal of concern has been

expressed concerning exposure to these agents via vaccinations,

there is no evidence that such fears are warranted.”

Vaccination is good

Preventing disease through proper vaccination is far

safer, easier and more economical than treating the

disease after the horse is already sick. Many diseases

horses encounter are preventable and vaccination is

solid insurance, not to mention an important biosecurity

measure to help prevent mass outbreak of disease.

In addition, some diseases, such as rabies, carry

zoonotic risk (meaning they can be passed between

animals and humans), requiring us to remain diligent

with vaccination. In fact, many of the equine diseases

we vaccinate against are deadly and treatment after the

disease is present is often unsuccessful.

Veterinarians recommend that all horses receive

some combination of vaccines every year. These recommendations

are based on the American Association of

Equine Practitioner’s (AAEP) core and risk-based vaccination


Horses being transported or shipped, as well as

in competition, are particularly susceptible to infectious

disease because they come into contact with

many horses and environments, and often are immunocompromised

(immune system cannot respond

appropriately) due to the stress of travel, training and

competing. This is representative of the United States

Equestrian Federation’s (USEF) recent decision to

implement a rule mandating that all horses be current

(within six months) on their EIV and EHV vaccines

prior to entering a USEF show.

If you own more than one horse, it’s important that

all horses be included in your vaccination program.

Just a single unprotected horse in a herd can serve as

a reservoir of infection to others. Devastating diseases

such as WNV and EEE/WEE are not discerning – any

horse exposed to infected mosquitoes is susceptible.

Particularly susceptible are those horses that are not

vaccinated or have weakened immune systems. Keep

in mind, senior horses may experience declining immune

function due to age – this is known as immunosenescence.

As a result, vaccination strategies for

senior horses may require further customization.

Importance of veterinarian-directed vaccination

Your veterinarian is critical in designing an appropriate

vaccination program for your horses and your

farm. He or she understands the individual needs of

your horse(s), as well as endemic and regional disease

considerations and need for vaccination.

Vaccination should normally generate some identifiable

response in horses; much like vaccination does

in people. Some injection site soreness, slight rise in

body temperature, and therefore some slight lethargy,

are normal responses. For these normal reasons, most

veterinarians recommend one to three days of rest after

vaccination. Though rare, vaccine reactions can occur.

As described, these reactions are generally mild and

self-limiting, but serious reactions such as anaphylactic

shock can occur. That’s why it is wise to have your

veterinarian administer vaccinations. He or she will be

prepared to handle a reaction if one should occur.

Most vaccine manufacturers offer vaccine guarantees

that help cover treatment costs should a properly vaccinated

horse get sick from the disease targeted by a specific

vaccine. Keep in mind these guarantees are only valid

when the vaccines are administered by a veterinarian.

If you are still uncertain about vaccination, talk with

your veterinarian. Equine infectious disease can strike

suddenly and be devastating to your horse – and others.

Vaccination is proven to be one of the safest and most

reliable life-saving measures you can take for your horse.

Get the facts on vaccination to help ensure a lifetime of

health and happiness for your equine companion. MeV

About the author

Earl Gaughan, DVM, DACVS, is an equine technical services

veterinarian with Merck Animal Health. He received his doctorate

of veterinary medicine from the University of Georgia and his

equine surgery residency at New York State College of Veterinary

Medicine, Cornell University. Dr. Gaughan has extensive experience

in both the classroom and field. He is a board-certified surgeon and

has authored numerous research articles and book chapters.

Copyright © 2016 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.Used with permission. | Issue 4/2016 4

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